Selective formation of formamidines or 7-aminomethylbenzoxazoles from unprecedented couplings between benzoxazoles and amines

Article abstract of DOI:10.1002/adsc.201100521

A method for the selective synthesis of amidines by ring-opening amination or of 7-aminomethylbenzoxazoles via a novel three-component-coupling reaction (arene, dichloromethane and amine) from benzoxazoles and amines was developed. Amidines could be further converted into 2-aminobenzoxazoles in high yield. Copyright

Full text of DOI:10.1002/adsc.201100521

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DOI: 10.1002/adsc.201100521
Selective Formation of Formamidines or 7-Aminomethylbenzoxa-
zoles from Unprecedented Couplings between Benzoxazoles and
Amines
Dingyi Yu,a Szehui Lee,a Yin Ngai Sum,a and Yugen Zhanga,*
a
Institute of Bioengineering and Nanotechnology, 31 Biopolis Way, The Nanos, Singapore 138669, Singapore
Fax: (65)-6478-9080; phone: (+65)-6824-7162; e-mail: ygzhang@ibn.a-star.edu.sg
Received: July 2, 2011; Revised: February 29, 2012; Published online: June 5, 2012
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/adsc.201100521.
Abstract: A method for the selective synthesis of
amidines by ring-opening amination or of 7-amino-
methylbenzoxazoles via a novel three-component-
coupling reaction (arene, dichloromethane and
amine) from benzoxazoles and amines was devel-
oped. Amidines could be further converted into 2-
aminobenzoxazoles in high yield.
nomethylation protocol to functionalize benzoxazoles.
To our surprise, the reaction between benzoxazole, di-
ethylamine and dichloromethane with CuCl catalyst
did not produce either the desired methylation prod-
uct 2 or the direct amination product 3 (Scheme 1).
Instead, two unexpected products, formamidine 4 and
the product of the 7-aminomethylation of benzoxa-
zole 5, were observed, as shown in Scheme 1. These
two important new products immediately triggered
our interest and here, we would like to report our
new findings about these coupling reactions between
benzoxazoles and amines.
Keywords: amidines; amination; aminomethylation;
benzoxazoles; three-component coupling
The amidine functionality represents an important
structural motif that turns up as a structural part in
Heterocyclic compounds are highly important because various drugs, such as quetiapine (anti-psychotic), ge-
of their abundance in numerous natural products and fitinib (anti-cancer) and dasatinib (anti-cancer) (see
pharmaceuticals of biological activity.[1] As a conse- Supporting Information for structures). Amidines are
quence, the direct functionalization of heterocyclic also important precursors used as reagents in the syn-
compounds has attracted tremendous attention, due thesis of heteroaromatics such as imidazoles and imi-
to the fact that the products are important synthetic dazolium salts,[5] as well as important units in coordi-
À
motifs, especially for C H bond activation and new nation chemistry, catalysis, biological recognition, sep-
carbon-carbon or carbon-heteroatom bond forma- aration and supramolecular chemistry.[6] The N-(o-
tion.[2,3] Recently, a useful aminomethylation method phenol)-formamidines are particularly important
was developed using a multiple-component reaction structural motifs with respect to their applications,
(AHA) among amines, dihalomethanes and terminal however, their synthesis still suffers from various diffi-
alkynes.[4] Since terminal alkynes and azoles have sim- culties.[6b,7] The ring opening amination of benzoxa-
ilar pKa values, we were interested to apply this ami- zole in Scheme 1 provides a simple and direct way to
Scheme 1. Multiple-component reaction between benzoxazole, dicloromethane and diethylamine with a CuCl catalyst.
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Selective Formation of Formamidines or 7-Aminomethylbenzoxazoles
Table 1. Optimization of the reaction conditions of the ring-
opening amination reaction with benzoxazole.[a]
Entry Catalyst (mol%) Solvent Time [h] Yield [%][b]
1
2
3
4
5
6
7
8
KCl (10)
KBr (10)
KI (10)
NH4Cl (10)
NH4Cl (10)[c]
NH4Cl (20)
NH4Cl (10)[d]
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
THF
6
6
6
6
6
6
6
6
6
6
2
6
6
6
6
6
6
6
6
40
42
40
50
50
70
70
47
93
93
92
68
60
75
72
83
85
65
70
0
ACHTUNGTRENNUNG(n-Bu)4NI (10)
9
CuCl (10)
CuCl (10)[c]
CuCl (20)
Cu2O (10)
10
11
12
13
14
15
16
17
18
19
20
Figure 1. Kinetic study of the ring-opening amination reac-
tion with benzoxazole. Reaction conditions: benzoxazole
(1.0 mmol), diethylamine (1.2 mmol), catalyst, CH3CN
(2 mL), room temperature. The reactions were quenched at
each time point by adding water. Yields given are of the iso-
lated product.
CuACHTUNGTRENNUNG(OAc)2 (10)
CuCl2 (10)
CuCl (10)
CuCl (10)
CuCl (10)
CuCl (10)
CuCl (10)
–
DMF
DMSO
toluene
CH2Cl2
obtained (Table 1). The production of formamidine 4
generally worked well in various common organic sol-
vents (Table 1). Interestingly, when the reaction was
carried out with other halide salts (10 mol%), such as
KCl, KBr, KI, (t-Bu)4NI and NH4Cl, formamidine 4
was also produced in moderate to good yields.
Among the halide salts tested, NH4Cl had the highest
catalytic efficiency. Compared to CuCl catalyst,
NH4Cl is less active, but with higher selectivity,
Figure 1. Even at 858C, the reaction with NH4Cl cata-
CH3CN 12
[a]
Reaction conditions: benzoxazole (1.0 mmol), diethyl-
ACHTUNGTRENNUNGamine (1.2 mmol), solvent (2 mL).
GC yield.
1.2 mmol DBU were added.
Reaction was conducted at 858C.
[b]
[c]
[d]
access the very useful N-(o-phenol)-formamidine unit. lyst gave up to 70% yield of 4 while no 5 was ob-
In addition, formamidines can also be directly con- served (Table 1 and Table 2).
verted to 2-aminobenzoxazoles efficiently.[3,8–10] Very
Using the conditions of CuCl (10 mol%) catalyst in
recently, Changꢁs group reported that the ring open- acetonitrile at room temperature for 6 h, it was found
ing amination of benzoxazole could occur in neat con- that cyclic, acyclic aliphatic secondary amines gave
ditions without catalyst, however, the reaction is slug- excellent yields in the range of 90–95% (Scheme 2).
gish in solution.[11] With that, the reaction conditions However, no reaction was observed for bulky amines,
in solution for the formation of 4 in Scheme 1 were such as diisopropylamine. Interestingly, when a pri-
screened. First of all, it was found that the reaction mary amine (n-butylamine) was tested in this reac-
temperature is crucial to the chemical selectivity of tion, the ring-opening amination product was also
this reaction. When the reaction was conducted at produced and was stabilized by coordination to
room temperature or below 608C, formamidine 4 was copper metal to form a complex 6 which could be re-
exclusively produced as the only product (Table 1 and crystallized. The single crystal X-ray structure of 6 is
Table 2). This ring-opening amination reaction of shown in Figure 2. With half an equivalent of copper
benz
ACHTUNGTRENNUNGoxazoles and diethylamine was typically carried salt present in the reaction system, complex 6 was iso-
out with CuCl as catalyst to afford N,N-diethyl-N’-(2- lated in quantitative yield (Scheme 3).
hydroxyphenyl)formamidine (4) in good to excellent
The development of this reaction not only presents
yield (Table 1). Further study showed that dichloro- a simple method to synthesize formamidine from
methane was not involved in the reaction and addi- benzACTHUNTGRNEUNGoxazoles, but also provides new insight for the
tion of base had no effect on this reaction. Cu2O and direct oxidative amination of benzoxazole. Recently,
À
other copper salts, such as CuCl2 and Cu
G
also screened and a lower yield of 4 (60–75%) was azoles have attracted much attention.[3,8–10] Since 2009,
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Table 2. Optimization of the reaction conditions for the 7-
several groups have reported protocols for copper
salt-catalyzed direct amination of azoles with amines
under an oxygen atmosphere.[12] The reaction is be-
methylamination of benzoxazole.[a]
lieved to proceed via deprotonation of the relative
2
À
acidic sp C H bond of the heterocycles by a base or
metal catalyst, followed by oxidative addition, trans-
metalation and reductive elimination. At the same
time, Chang and others proposed a ring-opening
mechanism for metal-mediated (such as Ag, FeCl3)[13]
(OAc)2][8]
or metal-catalyzed [such as CoACHTNUGRTNENUG(OAc)2 or MnACHTUGNTRENNUGN
oxidative amination of benzoxazoles under acidic con-
ditions, and 2-aminobenzoxazolidine/amidine were
suggested as the key reaction intermediates. However,
the detailed reaction mechanism for this important re-
action, such as the role of acid additive, is still un-
clear, especially as more new metal-free catalyst sys-
tems emerge.[9,14] Hererin, we managed to understand
some key factors of the reaction by studying the con-
ditions for the formation of amidine 4 and the condi-
tions for the transformation of 4 to coupling product
3. As shown in Scheme 4, it was found that the model
reaction proceeds smoothly to give amination product
3e in good to excellent yield with metal salt or halide
salt catalysts in the presence of an acid additive (con-
ditions 2). However, without the acid additive, the re-
action produced a mixture of 3e and amidine 4e
under the same conditions (conditions 1 in Scheme 4).
This means that acid additives are essential for the
high yield of 3e as well as to inhibit the production of
4e. Another set of results (conditions 3 and 4,
Scheme 4) indicate that 4e was produced in high yield
with or without an acid additive in the absence of oxi-
dant. To understand the role of the acid additive in
this reaction, conditions for the transformation of 4e
to 3e were further studied (conditions 5 to 8,
Scheme 4). It was found that the acid additive is es-
sential for the conversion of 4e to 3e in these systems.
Interestingly, the CuCl catalyst was less active than
Entry Catalyst Base
Temp. [8C] Time [h] Yield [%]
4
5
1
2
NH4Cl
NH4Cl DBU
–
25
85
25
25
60
85
85
85
85
85
85
6
6
6
6
6
12
6
12
12
24
12
12
12
12
12
50
70
93
93
95
0
15
17
60
17
30
37
35
25
30
0
0
0
0
0
0
60
80
35
80
66
60
60
70
64
3
4
5
6
7
8
9
10
11
12
13
14
15
CuCl
CuCl
CuCl
–
CuCl
CuCl
CuCl
CuCl
CuCl
CuCl
CuCl
CuCl
CuCl
–
DBU
–
DBU
DBU
DBU
–
DBU
TEA
Cs2CO3 85
KOH 85
t-BuOK 85
KHMDS 85
[a]
Reaction conditions: benzoxazole (1.0 mmol), dichloro-
methane (1.1 mmol), diethylamine (1.2 mmol), catalyst
(0.1 mmol), CH3CN (2 mL).
[b]
GC yield.
CoACTHNUTRGNE(UNG OAc)2 or Bu4NI in the conversion of 4e to 3e.
With that, a revised reaction pathway is proposed for
the catalyzed oxidative amination of benzoxazoles
(Scheme 5). Benzoxazole is first activated by Cu(I) or
proton from NH4Cl.[8] This is followed by addition of
the amine to form intermediate 7. In this step, the
acid additive is not a determining condition. Inter-
mediate 7 is not stable and is quickly converted to
amidine 4 or oxidized to product 3. Here, the acid ad-
ditive plays a key role in promoting the transforma-
tion of 4 to 7 by protonation of the imine. Alterna-
tively, for copper catalysts, the formation of the orga-
nocopper intermediate 8 cannot be excluded.[15] An-
other pathway for the conversion of 4 to 3 is via the
phenoxyl radical intermediate 9. However, this path
way only happened when TEMPO was used as oxi-
dant.[10,16] The control experiment using TBHP as oxi-
dant and lutidine as base did not give any product of
3 (Supporting Information, Scheme S2).
Scheme 2. Ring-opening amination reactions with benzoxa-
zoles to form formamidines. Reaction conditions: benzoxa-
zole (1.0 mmol), amine (1.2 mmol), CuCl (0.1 mmol),
CH3CN (2 mL), room temperature, 6 h.
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Selective Formation of Formamidines or 7-Aminomethylbenzoxazoles
Figure 2. ORTEP structures of 4e (a), 5a (b) and 6 (c). Thermal ellipsoids are shown at 50% probability. Hydrogen atoms
have been omitted for clarity.[20]
Scheme 3. Ring-opening amination reaction of benzoxazole and n-butylamine. See Figure 2 for the X-ray structure of 6.
In Scheme 1, the formation of 7-diethylaminome- three-component coupling represents a novel and
thylbenzoxazole 5 is a new type of three-component simple route for arene aminomethylation.
coupling reaction.[4] Notably, the extension from al-
It is found that the selectivity of this reaction is a ki-
kynes to arenes is unprecedented both for AHA cou- netically controlled, Table 2. At lower temperatures,
pling and for AAA coupling reactions.[17] Aminome- only amidine product 4 was produced. When the reac-
thylation of arenes is one of the very important meth- tion temperature was raised to 858C, three compo-
ods for the introduction of the amine functionality. nent coupling product 5 was obtained as the major
Traditionally, aminomethylation could be achieved by product. The copper catalyst is essential to the cou-
the reaction of an amine with arylaldehyde precursors pling reaction. There was no aminomethylation prod-
or by oxidative coupling between an alkylamine and uct observed without a catalyst or with as NH4Cl cata-
arenes.[18] In addition, the Mannich-type reaction be- lyst. One equivalent of base also increased the yield
tween phenol, formaldehyde, and an amine is known; of the reaction. Generally, organic bases have slightly
however, it is only limited to phenol.[19] The current better performance than inorganic bases, possibly be-
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Scheme 4. Control experiments for the transformations of 1, 3e and 4e.
Scheme 5. Proposed reaction pathways for the benzoxazole amination reactions.
cause of their better solubility in the reaction system CH2Cl2 with Cu(I) catalyst would happen. Followed
(Table 2). It turned out that using 8,11-diazabicyclo- by ring-closing, the final product 5 is generated (Sup-
ACHTUNGTRENNUNG
There was no reaction observed for bulky amines, benzoxazoles and amines was developed and ami-
such as diisopropylamine (5f) and 2-methylbenzoxa- dines could be further converted into 2-aminobenzox-
zole (5e). In addition, no reaction was observed as azoles in high yield. While studying these conditions,
benzthiazole (5g) and benzimidazole (5h) were used new insights were gained for the reaction mechanism
instead of benzoxazole.
of the direct oxidative amination of benzoxazole. By
The plausible mechanism of this reaction may in- adjusting the reaction conditions, a novel three-com-
volve a reaction of phenol intermediate 4, which is ponent coupling reaction (arene, dichloromethane
generated by a ring-opening amination step. In fact, and amine) was developed. It represents a novel and
the ring-opening by-product 4 is always observed in simple route for arene aminomethylation. The de-
this reaction system (Table 2). Then, a Mannich-type tailed mechanism of this reaction and its further de-
reaction between phenol intermediate 4 and iminium velopment will be reported in due course.
intermediate which is generated from amine and
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Selective Formation of Formamidines or 7-Aminomethylbenzoxazoles
Scheme 6. Three-component coupling reaction with benzoxazoles. Reaction conditions: benzoxazole (1.0 mmol), dichlorome-
thane (1.1 mmol), amine (1.2 mmol), CH3CN (2 mL).
Experimental Section
Acknowledgements
This work was supported by the Institute of Bioengineering
and Nanotechnology (Biomedical Research Council, Agency
for Science, Technology and Research, Singapore).
General Procedure for Ring-Opening Amination
Reactions (4a as Example)
A mixture of benzoxazole (1.0 mmol, 119 mg), diethylamine
(1.2 mmol, 86.5 mg) and CuCl catalyst (10.0 mg, 10.0 mol%)
was charged in the reaction tube (10 mL) with 2 mL
CH3CN. After stirring at room temperature for 6 h, the mix-
ture was diluted with H2O (10 mL) and the aqueous layers
were extracted with diethyl ether (2ꢂ10 mL), dried over
Na2SO4 and concentrated to give the crude product which
was further purified by column chromatography on silica gel
(ethyl acetate/hexane=1: 4) to afford the corresponding
pure formamidine (4a). All products gave satisfactory spec-
troscopic data. New compounds were characterized with
NMR, HR-MS analyses.
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